1. Field of the Invention
The present invention relates generally to optical filters and particularly to cascadable Mach-Zehnder Interferometers (MZI) in optical filters. More particularly still, it relates to methods of forming asymmetric MZIs by adjusting the two Michelson Interferometer (MI) components of an MZI prior to combining them into an asymmetric MZI.
2. Introduction
Optical filters are desirable in modern optical communication transmission systems and networks. These optical systems and networks use Wavelength Division Multiplexed (WDM) signals, in which each data channel sends its data using a light with a particular wavelength. WDM signalling increases the transmission capacity while providing more flexibility by the possibility of wavelength connectivity, routing, etc. Optical filters that can combine or separate different wavelength channels, known as WDM multiplexers or demultiplexers, therefore are important components of these systems. WDM filters can be fabricated by variety of methods such as using thin film dielectric filters or array waveguide gratings utilizing planar optical waveguides. WDM filters also can be made by interferometers and more specifically by number of interferometers cascaded in prearranged configurations. One class of these interferometers based optical filters are Cascaded Asymmetric Mach-Zehnder Interferometers (MZIs). Asymmetric MZIs show a periodic response as a function of wavelength the period of which is a function of length difference between the arms of the interferometer. The longer the length difference the shorter the wavelength response oscillations and therefore the higher wavelength selectivity. Asymmetric MZIs when connected to each other as interleavers can multiplex or demultiplex a large number of optical signals with different wavelengths such as the standard ITU grid wavelengths. An optical filter interleaver can separate the odd and even wavelengths from a WDM signal with a number of wavelengths.
The MZIs that are constructed by optical fibers are called all fiber MZIs. All fiber MZIs are desirable because of their excellent compatibility with the transmission media, i.e. optical fibers, low polarization dependency and low insertion loss compared to other WDM filters fabricated by other technologies. However, the challenge in all fiber Asymmetric MZIs is to control the length difference between the arms of an all fiber asymmetric MZI, since the wavelength response depends on this length difference. To make an all fiber MZI, two optical couplers that are used as splitter and combiner in the Interferometer are prefabricated. The optical couplers in an all fiber MZI configuration are usually fused tapered couplers. In fabricating fused tapered couplers two optical fibers are brought into close proximity and are heated to certain degree while pulling the two fibers so that the cores of the optical fibers come close enough to each other such that they can start to transfer energy by evanescent field coupling.
The processes of making fused tapered couplers are relatively sensitive and have low yield to achieve a desired coupling, i.e., splitting, ratio. However making two optical fused couplers in an asymmetric interferometer configuration with accurate length differences in one process is even more challenging, resulting in all fiber asymmetric MZIs being a low yield fabrication process.
It is therefore desirable to have an alternative and lower cost method of constructing all fiber asymmetric MZIs.
The present invention provides an all fiber asymmetric MZI by first making a coupler with two arms in the form of a Michelson Interferometer (MI) and then adjusting the length(s) of the arms(s) of the MI by precision grinding and/or polishing until the desired wavelength response is achieved. While performing this process step on the MI, a very accurate estimate of the length difference between the arms is obtained from the analytical relations between the length difference and the wavelength response. It is also possible to measure the length difference between the two arms of the Michelson interferometer by high precision reflectometers such as HP model No 8504B with a resolution of better than 20 microns. To achieve an even better resolution than with a reflectometer, it is possible to use optical spectrum analyzers to compute the length difference from the spectral shape of the Mchelson interferometer wavelength response. By the latter method it is possible to adjust the length difference to within one micron precision.
After making the MIs with the desired length difference, a Mach-Zehnder Interferometer is made my connecting two MIs back-to-back. The total length difference of the resultant asymmetric MZI, therefore, is either the sum of the length differences of the two individual MIs or the difference of the two. In this way a systematic method of constructing and fabricating all fiber asymmetric MZIs with the desired length differences, ie. wavelength responses, with very good precision.